Characterization of immunopathological changes in the feather pulp of CVI988-vaccinated pullets challenged with a very virulent plus Marek’s disease virus strain

High load of oncogenic Marek's disease virus (MDV) DNA in the feather pulp (FP) as early as 21 days of age is a powerful criterion to predict the outcome of Marek's disease (MD) in apparently healthy chickens. The objective of this study was to elucidate the immunopathological changes in the FP of 21-day-old chickens that had been vaccinated with CVI988 vaccine (healthy), vaccinated and challenged with a very virulent plus (vv+) MDV strain 648A (well protected), or were unvaccinated and challenged with 648A strain (not protected) when compared to uninfected naïve chickens. Oncogenic MDV DNA load, histopathological and immunohistochemical evaluation of the lesions, and immunophenotypic characterization of the infiltrates by flow cytometry were examined. Our results demonstrate that 648A-infected unvaccinated chickens had a significant increase in the percentage of CD3+ T cells, mainly CD4+MHC-II+ cells and CD8+MHC-II+ cells, when compared to all other groups. They also had a significantly decreased number of CD8β+ T cells compared to all other groups. Infection with 648A reduced the percentage of macrophages, not only in the unvaccinated group but also in the CVI988/648A group. In addition, groups that were vaccinated with CVI988, regardless of the challenge status, had higher levels of CD8β+ T cells, suggesting that the vaccine has an enhancing effect on the CTL cells. Our results showed that the load of oncogenic MDV is highly correlated with the infiltration of CD4+MHC-II+ cells and provide further confirmation that FP is indeed an appropriate sample for the early diagnosis of MD.